The present invention generally relates to power rotary hand tools and more particularly to an improved shaft lock mechanism for the same.
Small rotary hand tools that have a generally cylindrical housing or case have been marketed for many years for use in carrying out various woodworking and metal working tasks by hobbyists as well as commercial artisans. Such rotary hand tools generally have a motor unit with a rotary output shaft extending from the nose end and often have a nose portion that is configured to connect to various accessories or attachments. Some of these rotary hand tools are somewhat larger and more powerful and are known in the building trade as spiral saws that use a side cutting bit to penetrate and to rapidly cut holes for electrical outlets, light fixtures and switches and the like in dry wall. Because  these tools are quite powerful even though they are relatively small, they are convenient to use on a jobsite or just about anywhere else where a source of AC power is available.
Because such power hand tools can be used to perform many tasks, artisans in the building trades use them extensively and generally give them rough treatment during use. Because these tools are often the subject of abusive treatment, they must be ruggedly built to last. These tools typically have a chuck mounted on the motor output shaft for retaining side cutting spiral saw bits, drill bits, grinding tools and the like, so it is necessary to hold the output shaft from rotting so that the chuck can be tightened or loosened to change bits.
These tools therefore are provided with a convenient shaft locking mechanism that generally comprises a button in the front portion of the housing that has a spring loaded locking pin that can be inserted into an opening in the output shaft when it is correctly positioned and the button is depressed. One of the desirable features of such tools is that they are powerful but not particularly heavy. Their relatively light weight is at least in part due to the fact that the housing is fabricated from a strong, but lightweight plastic material.
It can be appreciated that when the locking pin is inserted into the output shaft and a user applies a lot of force to tighten or loosen the chuck, there can be substantial stress applied to the portion of the housing where the locking pin mechanism is located. Users are also known to depress the locking button after power has been turned off, but before the shaft stops rotating, for the purpose of applying a braking force to the shaft. Using  the locking pin mechanism as a brake is not what the tool is designed for and can result in damage to the tool.